Lonely Planets

  they’ve collided, merged, fragmented, and morphed into new shapes

  and colors, no new ones have been born since the cosmos was very

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  L o n e l y P l a n e t s

  young. This is our universe’s one and only crop of galaxies. They’ll

  have to do, unless someday we, or someone, figure out how to make

  new ones. And they are all slowly aging.

  G A L A C T I C E C O L O G Y

  In the beginning, the Milky Way and all other galaxies were made only

  of the hydrogen and helium created in the big bang.* Picture the peri-

  odic table, fixture of every science classroom, in which the 103 ele-

  ments are arrayed in columns according to their chemical behavioral

  types. Now, imagine a chemistry class during the youth of our galaxy.

  The chart on the wall would have been small, just two boxes, one each

  for hydrogen and helium, the two simplest atoms. The only chemical

  reactions possible were the coupling and uncoupling of hydrogen

  atoms. And you think science classes today are dull. But, not to worry,

  such a class would have been quite impossible. Without more elements

  to work with, there would have been nothing with which to build the

  classroom walls or print the chart. Without a richer palette of atoms

  with which to build complexity, it’s hard to imagine life of any kind

  evolving, let alone scientists and bored chemistry students. Fortunately,

  the galaxies evolve chemically as they age, slowly gaining more heavy

  elements. Thus they increase their potential for interesting chemistry,

  which, if local experience is any guide, means greater potential for life.

  Where does this increasingly diverse array of chemical elements come

  from? From the metabolism and decay of stars. For galaxies are the

  fields where stars are born, live, and die, and stars are element factories

  where nuclear fusion creates all the chemical richness of the modern

  universe.

  Stars result from the collapse of giant, diffuse clouds of gas that float

  among the galactic spiral arms. That collapse, initiated by random

  breezes that blow through the galaxy, soon reaches a point where, once

  again, gravity takes over, accelerating the contraction. From the pres-

  sure of the collapse, it becomes ever hotter in the center of the cloud.

  Eventually the gas at the center is squeezed so tightly and the collisions

  are so violent that individual atoms can no longer hold each other off.

  They start to merge in nuclear fusion reactions. At that moment of

  *Purists will note that a sprinkling of lithium was also made in the bang, but no manic-depressives would evolve to take advantage of this for billions of years.

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  79

  nuclear ignition, a star is born. In the hot cores of stars, matter contin-

  ues its climb toward more complex structure. While it liberates the

  energy that makes the stars shine, nuclear fusion also builds new and

  heavier elements. Hydrogen is fused into helium, helium to carbon, and

  so on, filling out the periodic table of the elements.

  During its lifetime, each star maintains a balance between the inward

  force of gravity and the outward pressure of heat and radiation liber-

  ated by the nuclear reactor at its core. When a star reaches old age,

  having spent much of its hydrogen fuel, it can no longer hold this bal-

  ance. For the most massive stars, the end is dramatic and quick, a cata-

  clysmic supernova explosion in which a dying star can briefly outshine

  its entire galaxy. In the intense flash of a supernova, pressures and tem-

  peratures dwarf even those found inside a normal, healthy star, and still

  more new elements are made. The nickel in Earth’s metallic core, the

  silver hanging from your ears, and all the gold in Fort Knox were made

  in the creative flashes of stellar demise.

  In the calamitous explosions of stellar self-cremation, supernovae

  spread their remains far and wide, fertilizing the galaxy. Their ashes

  make their way into the great clouds of gas and dust that form raw

  material for new stars. Thus enriched, subsequent generations of stars,

  and their accompanying planets, start off with a greater endowment of

  the heavy elements. The greater the diversity of elements, the more the

  potential for building planets like Earth, made of iron, silicon, and oxy-

  gen. This also increases the possibility of complex chemistry, including

  carbon chemistry, the starting material for our kind of life. Our galaxy

  is a compost heap of elements, growing ever more fecund with the life

  and death of every star. Each successive stellar generation is born with

  the potential for more diverse planetary evolution and more complex

  chemistry. Our own Sun is probably a third-generation star.

  A galactic ecology is at work here. Just as a forest ecosystem has new

  plants and trees growing from the decayed remains of their predeces-

  sors, in a galaxy new stars are continually born of the gas and dust

  spread by their dying ancestors. Our galaxy is still young—a vast

  hydrogen sea of possibilities.

  A N C E S T R A L C L O U D

  If we’re got it right, parts of our origin story must be truly universal.

  Our version of events will be somewhat familiar to the children of

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  L o n e l y P l a n e t s

  other stars sprung from the same field of galaxies when we finally meet

  them and compare notes. We don’t exactly know where the universal

  part of Cosmic Evolution ends and the “local flavor” begins. We’re still

  trying to figure out how normal or rare are the events that have trans-

  formed planet number three from a rock into a reef. Yet, we do know

  where we got started as a distinct entity, separate from the universe at

  large, with our own local history and, now, culture. Our differentiation

  from everything else, our individuation, began some 4.6 billion years

  ago, when the universe was about two-thirds its current age, with the

  collapse of our ancestral cloud.

  Long before we became creatures living within a planetary biosphere,

  and just before we became a preplanetary disk of debris circling a

  young Sun, we were a molecular cloud floating in the arms of the Milky

  Way. Then, everything in our entire solar system was smoothly

  blended, drifting together in this diffuse cloud of gas and dust. You, me,

  the Elephant Man, the Dalai Lama, the neighbor’s barking dog, the

  flower shop down the street, the Great Wall of China, the core of the

  Earth, the Sun, and the planet Neptune: we were all one. Of course we

  still are, but back then it would have been obvious even without the aid

  of meditation, psychedelics, or quantum mechanics, as we were all

  ground up and comingled, all one and the same cloud.

  In the journey toward complex structure that matter had been fol-

  lowing since the bang, the next big step was chemistry. For interesting

  molecules to form, first we needed the more diverse array of atoms

  forged in the nuclear fires of living and dying stars. Then we needed the

  cool, nurturing environs of a molecular cloud, where solitary atoms

  could safely assemble. Later still, beneath the shelterin
g sky of at least

  one planet, the chemistry that began in the cloud would come to life.

  Our cloud was a potent brew of primordial hydrogen and helium

  heavily seasoned with the remains of dead stars: nitrogen, carbon, oxy-

  gen, and a smattering of other tasty elements. Ultraviolet light from

  nearby stars played with these atoms, sometimes knocking off their

  outer electrons. An atom with missing electrons is in desperate need of

  others to complete itself. Wounded atoms will band together, sharing

  and trading electrons, combining and rearranging themselves into mol-

  ecules. Where carbon was involved, these elements formed complex

  organic Tinkertoy structures that blew about in the cosmic breezes.

  The birth of our solar system was helped along by the ashes and the

  splashes of the Sun’s dying ancestors. In addition to seeding our cloud

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  81

  with heavy elements, the final spasms of spent stars sent shock waves

  through the cloud, squeezing it in some places and thinning it in others.

  The temperature in the cloud varied, and it was more dense in the

  cold spots. One of the cold dense spots, nudged by the hot breath of

  a nearby supernova, started the fateful collapse that led to our solar

  system.

  Once it got started, the collapse happened fast. For the first million

  years it was in free fall, accelerating until enough gas pressure built up

  in the center to resist gravity and slow things down. As it shrank, it

  started to develop a distinct shape, morphing from a roughly spherical

  cloud into a flattening disk.

  Again with the disks! Why always a disk? Some basic physics tells us

  why a collapsing cloud becomes a spinning disk. Like a break-dancer

  spinning on her head, pulling in her knees, our cloud spun faster and

  faster as it contracted. It had to. It’s a law: the conservation of angular

  momentum. The more a spinning object contracts, the faster it spins.

  Like other physical laws, there’s no penalty for breaking it—you simply

  can’t.*

  The spinning cloud began to flatten out into a disk, for the same rea-

  son that a spinning ball of dough tossed into the air by a pizza chef

  stretches out into a pie shape. The centrifugal force caused by the spin-

  ning motion flattens any fluid object—whether made of soft dough or

  diffuse interstellar gas and dust.†

  Finally, when things got hot enough at the center of the collapsing

  cloud, nuclear fusion began and the Sun turned on, heating and lighting

  the surrounding preplanetary disk.

  M A K I N G W O R L D S

  At this point the picture should sound familiar: this is the flattened,

  spinning disk described in the eighteenth century by Laplace with his

  nebular hypothesis. In the intervening centuries, and especially in four

  decades of space exploration, we’ve learned a lot more about how a

  disk became a system of worlds.

  *In fact there are huge rewards if you do succeed in breaking one of these laws. A Nobel Prize and maybe even an appearance on Letterman.

  †You can feel this same force by simply twirling your body. Your arms take flight, pulled outward by the same force that pulled the twirling preplanetary cloud into a disk.

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  The first step was a blizzard of snow made of metal, rock, and ice.

  When the collapse slowed, the disk began to cool off. Solid material

  condensed out of the cooling gas, just as snow condenses out of a cool-

  ing cloud of water vapor. In this case the snow was made of a wide

  range of materials, which varied with the changing temperatures at dif-

  ferent distances from the Sun. In the hot regions near the Sun it snowed

  flakes of metal and rock. Farther out, around the present orbit of

  Jupiter, it was cold enough for ices to form: both the familiar

  snowflakes of water ice that adorn winter on Earth and more exotic

  snows of frozen methane and ammonia.

  When these snowflakes collided, they stuck together, forming larger

  aggregates—dust bunnies of metal and rock near the Sun, and icy

  snowballs farther out. These continued to collide and stick, forming

  larger clumps, beginning the process of growth by accretion that even-

  tually created the planets. Between the time when all was gas and dust

  and the final formation of a small number of large planets, our solar

  system was a raging storm of billions of tiny planets orbiting the Sun—

  the planetesimals.

  As planetesimals grew to a kilometer in size, they began to hold

  together by their own self-gravity. They also started to yank one

  another around, gravitationally perturbing each other’s orbits. The

  near misses, which were more common than actual collisions, scattered

  orbits far and wide, churning up the disk and facilitating further colli-

  sions. The few planetesimals that, by chance, grew the fastest became

  the embryos of planets. Their gravitational hunger only increased as

  they grew, sucking in and consuming the lesser planetesimals.

  The last stages of this growth were violent and chaotic. As the plan-

  ets approached their final sizes, giant also-rans, the contenders that

  could have been planets, came hurtling down to Earth (and Mercury,

  Venus, etc.) at speeds of tens of thousands of miles per hour. These final

  giant impactors left a trail of destruction throughout the solar system,

  stripping Mercury of its outer rocky mantle, leaving Venus spinning

  backward, and knocking Uranus on its side. And in an event as propi-

  tious for us as it was random, a Mars-size protoplanet smacked into the

  young, still-forming Earth, splashing a massive ring of vaporized rock

  into Earth orbit, which quickly condensed to make our singular, giant

  Moon.

  This entire growth process, from snowflakes to planets, was largely

  complete in 100 million years, although the tail end, a rain of leftover

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  83

  debris from planetary construction, continued to terrorize the planets

  for several hundred million more.

  The end result of all of this snowing, growing, clumping, bumping,

  and grinding was our solar system as we see it today. The initial segre-

  gation of material by temperature, which made metal and rock near the

  Sun, and ice farther out, has been preserved. Four small planets of rock

  and metal (which we call the terrestrial, or Earth-like planets) orbit

  close to the Sun, and four planets dominated by gas and ice (which we

  call the Jovian, or Jupiter-like planets) live farther out where cooler

  temperatures prevail. And then there’s tiny Pluto, the largest denizen of

  the frozen farther reaches of our system, where the icy planetesimals

  that never accreted into larger planets still trace out their long, slow

  orbits.

  How universal is the planetary part of our story? Everything we’ve

  been able to glean about how the planets formed here leads us to

  believe that when the Sun was born, the Earth and other planets were

  the inevitable aftermath. So, it seems, planets like ours should be a nat-

  ural by-product of starbirth, at least for stars that f
ormed under the

  same conditions as the Sun. But how rare, or commonplace, were these

  circumstances of birth? In a later chapter I’ll report on recent discover-

  ies that are finally shedding some light on that all-important question.

  What we do know is that at least once the expanding, cooling, coalesc-

  ing, condensing, accreting mess of the early universe was formed into a

  sheltering tide pool of a planet, sufficiently protected from the pounding

  of the cosmic surf to harbor the delicate chemical structures needed for

  the next steps in the long climb of matter into life.

  T H I S I M M O R T A L C O I L

  Now we enter the planetary phase of the cosmic fable, where matter

  made the most astounding leap yet. The surface of a planet can be a

  good place for elements and simple molecules to get together, try new

  variations on their structural themes, and make ever more complex

  molecules. Especially if, as was this particular planet, the third stone

  from a third-generation star, it is blessed with a sprinkling of holy

  water rich in carbon, nitrogen, oxygen, sulfur, and phosphorus—the

  “biogenic elements.” It also helps if, when the music stops after the

  random accretionary dance, your planet winds up at a healthy distance

  from the irradiating glow of its newborn star. If a planet is not so hot

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  that complex chemicals are ripped apart by the frenzied molecular col-

  lisions that constitute heat, and not so cold that chemistry itself grinds

  to a halt in the atomic lethargy of frigid matter, then things could get

  interesting. Better yet, if you orbit at a distance where, with the help of

  some “greenhouse warming,” surface temperatures allow for plentiful

  liquid water, you probably can’t go wrong.

  The young Earth had no shortage of simple organic molecules, the

  building blocks of terrestrial life. They may have been made here on

  Earth out of water and methane, using energy from lightning, ultravio-

  let sunlight, submarine volcanoes, or the explosive shock waves of

  falling comets.* Or they could have come from outer space. Heaps of

  organic-rich interstellar dust made their way to Earth, frozen in icy

  comets.

  The young Earth was awash in organics. In the early, warm oceans,

  our organic molecular forebears played at combining and recombining